Conically shaped screenless internals for radial flow reactors

ABSTRACT

An apparatus for contacting a bed of particulate material with a cross flowing fluid, which maintains the bed of particulate material within a retention volume. The apparatus includes partitions for retaining particles, with apertures disposed within the partitions. The apertures are covered by shrouds that extend above the edges of the apertures to prevent solid particles from spilling through inlet apertures.

FIELD OF THE INVENTION

This invention relates to the field of fluid particle contact and to anapparatus for contacting fluids and particles. More specifically, thisinvention relates to a moving bed of particles with a cross-flowingfluid.

BACKGROUND OF THE INVENTION

A wide variety of processes use radial flow reactors to provide forcontact between a fluid and a solid. The solid usually comprises acatalytic material on which the fluid reacts to form a product. Theprocesses cover a range of processes, including hydrocarbon conversion,gas treatment, and adsorption for separation.

Radial flow reactors are constructed such that the reactor has anannular structure and that there are annular distribution and collectiondevices. The devices for distribution and collection incorporate sometype of screened surface. The screened surface is for holding catalystbeds in place and for aiding in the distribution of pressure over thesurface of the reactor to facilitate radial flow through the reactorbed. The screen can be a mesh, either wire or other material, or apunched plate. For a moving bed, the screen or mesh provides a barrierto prevent the loss of solid catalyst particles while allowing fluid toflow through the bed. Solid catalyst particles are added at the top, andflow through the apparatus and removed at the bottom, while passingthrough a screened-in enclosure that permits the flow of fluid over thecatalyst. The screen is preferably constructed of a non-reactivematerial, but in reality the screen often undergoes some reactionthrough corrosion, and over time problems arise from the corroded screenor mesh.

The screens or meshes used to hold the catalyst particles within a bedare sized to have apertures sufficiently small that the particles cannotpass through. A significant problem is the corrosion of meshes orscreens used to hold catalyst beds in place, or for the distribution ofreactants through a reactor bed. Corrosion can plug apertures to ascreen or mesh, creating dead volumes where fluid does not flow.Corrosion can also create larger apertures where the catalyst particlescan then flow out of the catalyst bed with the fluid and be lost to theprocess increasing costs. This produces unacceptable losses of catalyst,and increases costs because of the need to add additional makeupcatalyst.

The design of reactors to overcome these limitations can savesignificantly on downtime for repairs and on the loss of catalyst, whichis a significant portion of the cost of processing hydrocarbons.

SUMMARY OF THE INVENTION

A solution to the above problem is to design a catalyst retentionapparatus wherein the fluid is allowed to freely flow across thecatalyst bed, while the catalyst is maintained in a catalyst retentionvolume. The present invention provides for a cross-flow apparatuscomprising an inlet partition and an outlet partition for defining theparticle retention volume, or catalyst bed. The inlet partition furtherhas apertures defined therein, where the apertures are open holes ofsize sufficient for the solid catalyst to pass through. The inletpartition further includes a plurality of shrouds, wherein the shroudscover the inlet partition apertures such that gas can flow in throughthe apertures and under the shrouds before passing through the catalystbed. The shrouds are sized and positioned to prevent solid particlesfrom flowing through the inlet apertures.

Other objects, advantages and applications of the present invention willbecome apparent to those skilled in the art from the following detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a radial reactor inlet partition with conical shrouds;

FIG. 2 is a single shroud on an inlet partition covering a singleaperture;

FIG. 3 is a single shroud having a pyramidal shape;

FIG. 4 is a cross-section of a portion of the inlet partition and asingle shroud covering an inlet aperture; and

FIG. 5 is a cross-section of a portion of the inlet partition and asingle blister of another embodiment.

DETAILED DESCRIPTION OF THE INVENTION

A problem exists with radial flow reactors where a catalyst flows downan annular region, and the annular region is defined by an innerscreened partition and an outer screened partition, which defines thecatalyst bed, or a particle retention volume for holding a granularsolid. A fluid, usually a gas, flows across the partitions and catalystbed, reacting with the catalyst to produce a product fluid, also usuallya gas. The reactor holds the catalyst in with screens where the gasflows through. The partitions need holes sufficiently small to preventcatalyst particles from passing, but the holes are subject to pluggingand the subsequent creation of dead spaces where the gas doesn't flow,and the waste of catalyst that is not used. The screens in thepartitions are also subject to erosion and corrosion, creating holesthat allow for catalyst to spill out, which is prevented by the presentinvention.

The apparatus can also be an adsorber for adsorbing a constituent fromthe fluid flowing over a granular solid adsorbent. This includes anapparatus where the adsorbent is loaded and does not flow through theadsorber, but is held in place by the inlet and outlet partitions whilefluid flows over the granular adsorbent. The apparatus of the presentinvention is oriented for the downward, or in the direction of gravity,flow of a solid through the apparatus with the cross flow of a gas, andaccordingly, the use of the terms downward and upward are in referenceto directions relative to the direction of gravity.

The retrofitting of existing radial flow reactors provides for a methodof improving the reactors by using a screenless inlet partition. Ascreenless inlet partition provides for large openings for a fluid,usually a gas, to flow through the partition. However, large openingsalso provide for the granular solid in an adsorber or reactor to flowout. To prevent the spillage of solid through the openings, a cover tothe openings is provided on the inlet partition.

The present invention is an apparatus for supporting a granular solid ina cross-flow system, where the apparatus comprises an inlet partition,an outlet partition, and a plurality of inlet shrouds. The inletpartition has a plurality of apertures defined therein, where eachaperture has a lower edge and an upper edge, the outlet partition has aplurality of apertures defined therein, where each aperture has a loweredge and an upper edge, and where the inlet and outlet partitions arespaced to define a particle retention volume for holding a granularsolid. The inlet partition further includes a plurality of shrouds,where each shroud covers at least one inlet aperture to prevent thespillage of granular solid from the apparatus.

The inlet partition of a preferred embodiment is shown in FIG. 1, wherethe inlet partition 10 is a cylindrical structure for a radial flowreactor. The outlet partition is a larger cylindrical structure thatsurrounds the inlet partition 10 and forms the particle retention volumebetween the inlet and outlet partitions. The inlet partition comprises aplurality of apertures that are covered by shrouds 30 having a conicalshape, with the apex of the conical structure pointing upward and theopen end of the shroud facing downward, and with the shroud affixed tothe inlet partition 10 over an aperture, to prevent catalyst fromspilling out of the inlet apertures. The conical structure for theshroud 30 is chosen as it provides a shape that allows solids to flowover the shroud and through the reactor, while preventing the solidsfrom spilling through the inlet apertures. As shown in FIG. 2, theshroud 30 is affixed to the inlet partition 10 at a position above anaperture 20. The base of the shroud 30 is the lower edge 70 of theshroud and extends to a distance at least as low as the lower edge 110of the aperture 20.

The shroud 30 extends away from the inlet partition at an angle betweenabout 1 degree and about 85 degrees, with a preferred angle betweenabout 10 degrees and about 50 degrees. The shroud 30, having asubstantially conical shape, is not required to have the axis of thecone lie along the inlet partition 10, but is a section of a cone, andcan have the axis of the cone extend at an angle to the inlet partition10 to achieve a chosen steepness of the shroud, while maintaining asufficient breadth to cover an inlet aperture 20.

The opening angle of the cone is the vertex angle made by a crosssection through the apex and center of the base of the cone. Half theopening angle for the cone is the angle, θ, that the cone extends fromthe inlet partition into the particle retention volume. The shroud has aheight that extends from the base of the shroud to the apex of the coneshaped shroud. A cone is just a pyramid with a circular cross section.As such, pyramidal shapes are also useful for the shrouds, as shown inFIG. 3. For purposes of this invention, the shroud 30 comprises aconical, or pyramidal, section that has been cut wherein the cut extendsthrough the annex of the shroud 30 to the base of the shroud 30. Thebase of the shroud 30 makes up the lower edge 70, while the edge formedfrom the cut through the conical section is the upper edge for thisinvention. With respect to the apertures 20 in the inlet position 10,the upper edge 180 refers to the edge above the midline of the aperture20, and the lower edge 110 refers to the edge below the midline of theaperture 20.

During the filling process of the reactor, the solid fills the particleretention space, and some of the solids flow up into the void volumecreated between the inlet partition and the shrouds. The backfilling ofthis volume can create a loss of catalyst if the catalyst is allowed toflow through the inlet aperture 20. Avoiding the loss of catalystresults in significant savings as catalyst is one of the mostsignificant costs in a petroleum refinery. The spilling of catalystthrough the aperture 20 is avoided if the lower edge of the aperture isat a height above the lower edge of the shroud by a distance determinedby the angle of repose, φ, of the granular catalyst. A cross-section ofan inlet partition aperture 20 with a shroud 30 is shown in FIG. 4. Theangle of repose, designated by the number 50, is a property ofparticulate solids. When bulk particles are poured onto a horizontalsurface, a conical pile will form, and the angle between the edge of thepile and the horizontal surface is known as the angle of repose. Theangle is related to physical properties of the material, such asparticle size and shape, density, and the coefficient of friction of theparticles.

Preferably, the distance, or height, of the aperture lower edge abovethe base, or lower edge, of the shroud is determined according to theequation, as follows:d=L*sin(θ)*tan(φ),where L is the length of the shroud, θ is the angle the shroud extendsaway from the inlet partition, and φ is the angle of repose for thegranular solid. The length of the shroud, L, is the length from the apex60 of the shroud attached to the inlet partition 10 spanning to thelower edge 70 of the shroud extending into the particle retentionvolume.

The angle 40, θ, is between about 10 and about 50 degrees from thevertical and preferably is between about 20 and about 35 degrees, with amore preferred angle between about 25 and about 35 degrees. The angle ofthe shroud is preferably chosen to provide at least the same contactarea between the inlet gas and the surface of the bed of granular solidexposed under the shroud, as the surface area of a screened aperture.

The angle 40 of the shrouds 30 is chosen to minimize holdup of thegranular solid as the solid flows through the apparatus. It has beenfound that the uppermost shrouds preferably are of a steeper angle thansuccessive shrouds lower in the apparatus. The uppermost shroudspreferably should be oriented at an angle from vertical between about 1degree and about 20 degrees.

In one embodiment, the shrouds are arrayed in a staggered arrangement asone progresses down the inlet partition, as shown in FIG. 1. The use ofa staggered arrangement limits the amount of channeling of catalystbetween the shrouds as the catalyst progresses through the reactor. Fora radial reactor, the staggered arrangement can comprise havingsuccessive lower levels of shrouds positioned, from a circumferentialposition, between the shrouds of a higher level. The positioning doesnot need to be centered between two shrouds above, but can be chosen tobe at any circumferential position between two shrouds above.

In another embodiment, the apparatus further comprises an outletpartition having apertures. The apertures are not covered by a screen,but are covered by shrouds in a manner similar to the inlet partition.The shrouds have a conical or pyramidal shape and are affixed to theoutlet partition above the outlet apertures and extend into the particleretention volume at an angle between about 1 degree and about 85 degreesfrom the outlet partition. The outlet shroud lower edge extends to atleast the lower edge of the outlet apertures.

During the filling process of the reactor, the solid fills the particleretention space, and some of the solids flow up into the void volumecreated between the outlet partition and the outlet shrouds. Thebackfilling of this volume can create a loss of catalyst if the catalystis allowed to flow through the outlet apertures, as mentioned above forthe inlet aperture. The spilling of catalyst through the outlet apertureis avoided if the lower edge of the outlet aperture is at a height abovethe lower edge of the shroud by a distance determined by the angle ofrepose, φ, of the granular catalyst, and is also determined according tothe equation 1 above.

In another embodiment of the present invention, the apparatus comprisesan inlet partition, an outlet partition, and a plurality of inletblisters. The inlet partition has a plurality of apertures definedtherein, where each aperture has a lower edge and an upper edge, theoutlet partition has a plurality of apertures defined therein, whereeach aperture has a lower edge and an upper edge, and where the inletand outlet partitions are spaced to define a particle retention volumefor holding a granular solid. The inlet partition further includes aplurality of blisters, where each blister extends away from the inletpartition and away from the particle retention volume. Each blister hasa conical or pyramidal shape with the apex pointing in a downwarddirection, and the open end of the blister facing upward. In a preferredembodiment, each aperture on the inlet partition will have an upper edgeand a generally V-shaped lower edge, and will have a blister affixed tothe lower edge, with each blister extending outward from the inletpartition at an angle between about 1 degree and about 85 degrees. Eachblister has an open end with an upper edge, and the upper edge willextend to at least the upper edge of the aperture.

A cross-section of this embodiment is shown in FIG. 5. The lower edge 70of the blister 30 is affixed to the inlet partition 10 along the loweredge 110 of the aperture. In a preferred embodiment, the upper edge 60of the blister 30 will extend a distance above the upper edge of theaperture by a distance according to equation 1 above. And, it ispreferred that the angle, θ, which the blisters extend from the inletpartition, is between about 10 degrees and about 30 degrees.

The uppermost blisters preferably are of a steeper angle than successiveblisters lower on the inlet partition. The uppermost blisters preferablyshould be oriented at an angle from vertical between about 1 degree andabout 20 degrees.

While the invention has been described with what are presentlyconsidered the preferred embodiments, it is to be understood that theinvention is not limited to the disclosed embodiments, but it isintended to cover various modifications and equivalent arrangementsincluded within the scope of the appended claims.

1. An apparatus for supporting a granular solid in a cross-flow systemcomprising: an inlet partition having apertures, wherein each aperturehas a generally V-shaped lower edge and an upper edge; an outletpartition having apertures, wherein each aperture has a lower edge andan upper edge, and wherein the inlet partition and the outlet partitionare spaced to define a particle retention volume for holding a granularsolid; and a plurality of inlet blisters, each having an upper edge anda lower edge, each lower edge having a generally V-shape, and each inletblister lower edge affixed to the lower edge of the inlet aperture,where the inlet blister upper edge extends to at least the upper edge ofthe inlet aperture and wherein the uppermost inlet blisters extend awayfrom the particle retention volume space at an angle from vertical ofabout 1 degree to about 20 degrees, the remaining inlet blisters extendaway from the particle retention volume space at an angle from verticalof about 10 degree to about 30 degrees and the uppermost inlet blistersextend away from the vertical at a steeper angle than the remaininginlet blisters.
 2. The apparatus of claim 1 wherein the inlet blistershave an inverted conical shape.
 3. The apparatus of claim 1 wherein theinlet blisters have an inverted conical shape.
 4. The apparatus of claim1 wherein the inlet blisters have an inverted pyramidal shape.
 5. Theapparatus of claim 1 wherein the inlet blisters are staggered atsuccessive elevations on the inlet partition.